Phase-locked loops (PLLs) are used in data communications and telecommunications applications to lock onto the frequency of a signal. In particular, PLLs are often used in serializer/deserializer (SerDes) applications. A typical PLL includes a phase detector, a loop filter, and a voltage controlled oscillator (VCO) that produces a VCO signal. The phase detector compares an input signal with a portion of the VCO signal and produces an output that indicates the relative phase of the two signals. The output is provided to the loop filter and in response, the loop filter produces a voltage that causes the phase of the VCO signal to move closer to the input signal. A portion of the VCO signal is then looped back to the phase detector in a continuous process. When the loop is locked, the frequency and phase of the VCO signal track the frequency and phase of the input signal.
An important characteristic of a PLL is its ability to lock the VCO signal to the input signal even when the initial frequency of the VCO signal is far away from the frequency of the input signal. For example, the frequency of the input and VCO signals may be far apart when a PLL is first powered up, when there is a change to the applied input signal frequency, or when the input signal is reapplied to the PLL after a long absence.
A central component in ensuring signal lock is the phase detector. There are generally two classes of phase detectors, analog and digital. With a digital phase detector, if the frequency difference between the input and VCO signals is too large (typically greater than 1%), it is difficult for the PLL to achieve lock. Separate frequency detectors have been added to PLLs with digital phase detectors to ensure that the PLLs are able to achieve lock when the frequencies of the input and VCO signals are far apart (i.e., outside the capture range of the digital phase detector). A frequency detector typically uses an external reference clock to measure the frequency of the VCO signal. The measured frequency of the VCO signal is used to determine. whether or not the frequency detector should assert control over the VCO signal. If the frequency detector is to assert control over the VCO signal, then the frequency detector is used to pull the frequency of the VCO signal close to a pre-established target frequency. Once the frequency of the VCO signal is within the capture range of the phase detector, control of the VCO is switched over to the phase detector.
Many different techniques for frequency detection in PLLs have been developed. Although these techniques work well, there is still a need for a flexible frequency detection technique that supports a wide range of target and reference clock frequencies and that asserts control over the VCO signal at the proper time.